Lecture 23 Transport 2

Aphid-Based Insights into Plant Sugar Transport

• Early discoveries about plant sugars came from studying aphids.
  • Aphids pierce phloem, over-ingest sap and excrete sugary "honeydew" (also called "sundew").
  • Researchers vaporised aphids and analysed this excretion to identify phloem composition.
  • Illustrates: plants vigorously protect sugars; specialised herbivores evolve work-arounds.

Big Picture: Two Tasks in the Lecture

• Production of sugar (photosynthesis).
• Transport of sugar (phloem translocation).

Photosynthesis – Core Concepts

• Overall reaction (highly simplified):
  6\,CO2 + 6\,H2O + \text{light} \;\rightarrow\; C6H{12}O6 + 6\,O2
• Converts low-energy, oxidised carbon (CO₂) into high-energy, reduced carbon (carbohydrates).
• Drives global carbon cycle, agriculture, biofuels, and ultimately most heterotrophic life.

Two Inter-linked Phases

1. Light Reactions
   • Occur in chloroplast thylakoid membranes.
   • Use photons + water to produce high-energy compounds:
     \text{ADP} \;\rightarrow\; \text{ATP}
     \text{NADP}^+ \;\rightarrow\; \text{NADPH}
   • Release O_2 as a by-product.
2. Calvin Cycle (Carbon-Fixation Reactions)
   • Stroma of chloroplasts; cyclic enzymatic pathway.
   • Uses ATP and NADPH to reduce and assemble carbon.
   • Net output (per two turns) = one C_6 sugar (e.g.
     glucose, fructose).

Leaf Internal Anatomy & C₃ Photosynthesis

• Standard "C₃ leaf" (majority of temperate species):
  • Upper epidermis → palisade mesophyll (dense chloroplasts → high light capture).
  • Spongy mesophyll (air spaces, gas exchange, water interface).
  • Stomata concentrated on lower epidermis → minimise transpiration under direct sun.
• First stable Calvin-cycle product = 3-carbon molecule → name "C₃".

Enzyme Spotlight – RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

• Most abundant protein on Earth.
• Catalyses addition of CO_2 to a 5-C acceptor (RuBP).
• Fault: similar affinity for O_2 → photorespiration.
  • When stomata close (hot/dry), internal CO2 ↓, O2 ↑.
  • RuBisCO fixes O_2, generating toxic/energy-wasting products.

Evolutionary Work-Around 1 – C₄ Photosynthesis

• Kranz anatomy: concentric rings around vascular bundle.
  • Outer ring = mesophyll (light reactions, initial CO_2 capture).
  • Inner ring = bundle sheath (Calvin cycle).
• First fixation by PEP carboxylase (insensitive to O_2).
  • Forms 4-C acid (oxaloacetate → malate).
  • 4-C acid shuttled to bundle sheath; decarboxylated → local CO_2 ‘pump’.
• Benefits:
  • High photosynthetic efficiency when stomata are partially/fully closed.
  • Dominant in tropical/savanna grasses (maize, sugarcane).
• Costs:
  • Extra ATP needed to regenerate PEP and shuttle acids.
  • Less competitive in cool, moist, high-CO_2 environments.
• Has evolved independently ≈ \sim30 times across plant lineages.

Evolutionary Work-Around 2 – CAM Photosynthesis (Crassulacean Acid Metabolism)

• Temporal separation rather than spatial.
  • Night: stomata open; CO_2 fixed into 4-C malic acid and stored in vacuoles.
  • Day: stomata closed; light reactions supply ATP/NADPH; malate decarboxylated → Calvin cycle runs internally.
• Strong water saving; common in succulents, cacti, pineapple.
• Trade-off: limited malate storage → lower maximum photosynthetic rate.

Morphological Water-Saving Tricks

• Rolled/needle-like leaves (Triodia, Spinifex, conifers):
  • Inner "crypt" concentrates CO_2 and reduces vapor loss.
  • High volume : surface ratio.
• Thick cuticle, sunken stomata, reflective hairs and waxes.

Stomatal Regulation – Environmental Triggers

• Soil water potential ↑ → stomata open (no drought stress).
• Leaf internal CO_2 ↓ (actively photosynthesising) → stomata open.
• High light intensity (if water ample) → open.
• High temperature (boosts respiration \Rightarrow CO_2 demand) → open until water stress overrides.
• Circadian rhythms:
  • Typical C₃/C₄: open day, close night.
  • CAM: reverse.

Phloem – The Sugar Highway

Structure

• Continuous living pipeline parallel to xylem.
• Sieve-tube elements: elongated, no nucleus, sieve plates at ends.
• Companion cells: sister cell (from same mother cell) with nucleus + abundant mitochondria; handles loading/unloading & metabolic control.
• Associated parenchyma for storage/support.

Developmental Coupling

• Vascular cambium produces xylem inward, phloem outward.
• In stems: phloem outer side; xylem inner.
• In leaves: vein flips when entering blade → phloem faces lower epidermis (consistent with outside-of-stem position).

Source–Sink Dynamics

• Source = tissue where \text{sugar production} - \text{utilisation} > 0.
• Sink = tissue with net sugar demand.
• Example summer tree:
  • Leaves (source) → roots, fruits, growing shoots (sinks).
• Example early spring deciduous tree:
  • Roots/stem starch → hydrolysed to sucrose (source).
  • Buds & developing leaves (sink).
  • Demonstrates bi-directional sap flow potential.

Sugar Maple Case Study

• Sugar maple aggressively mobilises root reserves at thaw.
• Sap exudes rapidly when trunk is tapped → maple syrup industry.
• Ecological angle: rapid leaf flush out-competes slower neighbours but increases vulnerability to sap feeders.

Pressure-Flow (Münch) Mechanism of Phloem Translocation

1. Active loading of sucrose into source sieve tubes (uses ATP in companion cells).
2. Raises osmotic concentration → lowers water potential.
3. Water moves in from adjacent xylem by osmosis → builds positive pressure (turgor).
4. Pressure gradient pushes sap through sieve plates toward sinks.
5. Active unloading at sink removes sucrose; local water potential rises.
6. Water exits back to xylem → recycled up to leaves.

Allocation Logic

• Phloem chooses pathway with greatest pressure differential (fastest unloading sink).
  • Rapidly growing fruit may out-compete a slower-growing branch tip.
• No conscious choice—purely emergent from physical laws & metabolic rates.

Key Numbers, Terms & Molecules

• 3-C = 3-phosphoglycerate (first Calvin product in C₃).
• 4-C = oxaloacetate/malate (first product in C₄ & CAM).
• 5-C = RuBP (Calvin acceptor).
• 6-C = glucose/fructose (transported mainly as sucrose, a 12-C disaccharide).
• RuBisCO = Ribulose-1,5-bisphosphate carboxylase/oxygenase.
• PEPCase = Phosphoenolpyruvate carboxylase.

Ethical, Climatic & Practical Implications

• Understanding photosynthetic pathways guides crop breeding (e.g., engineering C₄ traits into rice for drought resilience).
• Improved phloem knowledge aids pest management (aphid control) and nutrient optimisation.
• Maple syrup industry = direct exploitation of phloem dynamics.
• Global climate change (lower atmospheric CO_2 historically vs present) highlights evolutionary constraints on RuBisCO efficiency.

Integrative Take-Home Messages

• Photosynthesis captures solar energy; phloem distributes it as chemical energy.
• Evolution produced anatomical (C₄), temporal (CAM), and structural (rolled leaves, needles) solutions to RuBisCO’s oxygen dilemma and water scarcity.
• Phloem transport is an energy-assisted, pressure-driven conveyor from sources to dynamic sinks, reversing direction seasonally.
• Coupling of xylem & phloem allows plants to recycle water while moving sugars, elegantly meshing physical physics with biological control.